Outline/Perimeter Shells

I have been using 3 outline shells on my objects. I was under the impression that 3 was the optimum for strength and finish.
I'm now not at all sure if that is true, I understand that the strength comes from the infill and not the number of vertical walls so I wonder if the finish would be much different with just 2 Perimeters instead of 3.

Any thoughts?

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appjaws - Core XYUV Duet Ethernet Duex5
firmware 3.1.1 Web Interface 3.1.1
Ormerod 1-converted to laser engraver, Duet wifi
OpenSCAD version 2020.07
slic3r-1.3.0, Simplify3D 4.1.2, Cura-4.4.1

I've been using only 2 perimeters and haven't noticed any real difference in finish. It seemed to reduce print time as well.

I also have been using 2 perimeters and have made hollow cores to force a single shell middle wall in some solid prints. Some of the honeycomb etc patterns are not really optimal.

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Socrates ~ The Amsterdamman
slic3r-1.2.9

The strength comes from walls or infill depending what direction the force is applied. Torsional strength comes from the walls (a hollow drive shaft is almost as strong as a solid shaft), but the infill provides crush resistance. Wear resistance is a function of the material, finish and design geometry and will not be much affected by perimeters or infill - except a weaker part may wear less than a stronger part because it deforms to better accommodate a mating part. I have used 2 perimeters and 30% or 40% infill for almost everything to date and find it is strong enough for structural designs I have made and the Ormerod parts and gears, and my gut feeling is that increasing this will not improve the strength of the part by very much. Perimeters usually take longer to print than infill, but there is an exception with thinnish vertical walls, where (say) a 3mm wall will print faster as a solid piece with 3 perimeters per side at 0.5mm extrusion width (i.e. the wall is made entirely from 6 perimeters) than the same wall with 2 perimeters that thus has 4 outline perimeter extrusions plus a narrow infill. The many direction changes of the infill keeps the print speed down so it takes longer than printing 2 additional perimeters.

Dave

Thank you Dave,
Very clear, I will stick to 2 perimeters from now on and also reduce the infill percentage.

Just one question, do you have any problems with nuts and bolts tightening with 2 perimeters and 30-40 percent infill?

Paul

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appjaws - Core XYUV Duet Ethernet Duex5
firmware 3.1.1 Web Interface 3.1.1
Ormerod 1-converted to laser engraver, Duet wifi
OpenSCAD version 2020.07
slic3r-1.3.0, Simplify3D 4.1.2, Cura-4.4.1

Quote
appjaws1
Thank you Dave,
Very clear, I will stick to 2 perimeters from now on and also reduce the infill percentage.

Just one question, do you have any problems with nuts and bolts tightening with 2 perimeters and 30-40 percent infill?

Paul

Yes, so I often use large washers. Note that this is nothing to do with perimeters or infill, because with 2 perimeters the bore of the hole is a solid plastic tube 1mm thick, which will cover the size of an M3 bolt head or nut, so extra perimeters or denser infill will not help. Holes will print smaller than designed, so design holes with an oversize so they print almost the right size rather than drilling out after the print, because the drill will remove much of that solid bore.

The plastic we usually use will deform easily and so is squashed by high pressures (PLA a bit more so than ABS). Other less commonly printed plastics such as nylon might be better. Countersunk bolts especially will push the plastic sideways and loosen, and so are generally a bad idea if you need a strong fastening. I do however find that nut traps work very well (Here you have to experiment to find the oversize that will print out the correct size, because of course you cannot easily enlarge them afterwards - though on occasion I have heated a nut so that it melts into its nut trap).

Having said that, I print in ABS which you can glue together with acetone to form a chemical weld that is as strong or stronger than a solid part, and I use that to fasten parts together whenever they will rarely need taking apart. If I subsequently find I do need to take them apart, I cut them apart and print a new set. Obviously that does not cover occasions when parts must be fastened to another material or fastening parts that move.

Dave